Sealed batteries are largely classified into cylindrical batteries, prismatic batteries and pouch-type batteries depending on the shapes and materials of housing cases in which an electrode assembly that is the power generation element is accommodated. In particular, cylindrical batteries have found wide use in applications such as electric tools, power-assisted bicycles and electric vehicles. Thus, cylindrical batteries are required to exhibit high safety even when used under severe conditions.
In a cylindrical battery, a sealing unit includes safety measures such as an explosion-proof valve, a current interrupt device and a PTC (positive temperature coefficient) element. Patent Literatures 1 and 2 are example documents which disclose such sealing units. A sealing unit disclosed in Patent Literature 1 will be described below with reference to FIG. 5.
The sealing unit illustrated in FIG. 5 is composed of an aluminum case 56 in the form of a dish and, fitted therein, a stack including an external terminal cap 51, a PTC element 52, an aluminum foil 53 and an aluminum foil 55. A central portion of the aluminum foil 53 is connected to the aluminum foil 55 disposed farther inside the battery than the aluminum foil 53, and an outer peripheral portion of the aluminum foil 53 is insulated from an outer peripheral portion of the aluminum foil 55 by an insulating gasket 54. The aluminum foil 55 has a thin portion 55a. If the pressure inside the battery is raised, the thin portion 55a ruptures to interrupt the current path within the sealing unit. In this manner, the current interrupt device makes use of a rupture of part of the metal members by the action of the pressure inside the battery.
Patent Literature 2 discloses that a thin portion of an internal terminal plate that will rupture to interrupt the current is formed polygonal in cross section, and also discloses that the thin portion is curved at its tip in cross section. These configurations lead to a relaxation of the stress experienced by the thin portion when the internal terminal plate is welded to the valve member, and thus prevent the thin portion from deformation. Consequently, the variation in actuation pressure of the current interrupt devices is reduced.